Whether the prevention works or not I can't say, but this is mixing apples
and oranges so to speak. This post is referencing the energy in the strike.
The theory behind the porcupine is to *prevent* the strike by dissipating,
or rather letting the charge leak off before it can build up. The energy at
this stage is much lower than that contained in the strike.
As the storm moves over the landscape localized (size depends on how you
define localized) charged areas move across the surface with corresponding
charged areas in the clouds. These areas may be well separated horizontally
by as much as 10 miles. The charged area on the ground doesn't even have to
be under the storm. I've seen lightning strike the ground over 10 miles from
the cloud/storm with the hit out in bright sunlight.
We have to define charged areas as there are charge gradients across the
surface most of the time. So some how we need to define an elevated charge
versus the day-to-day stuff, so I'm going to stick with generalities..
The charged areas are *probably* on average on the order of 5 to 10 acres
with a *substantial* gradient across the area. These charged areas can
*flow* or move at substantial speeds. Typical is going to be the average
speed of the storm, but they can move faster or slower. Due to soil
resistance even a major strike is not going to entirely neutralize the
charge. More than likely a substantial charge will remain.
So we have not only a charge build up, but it's also a moving target which
we generally think of as being stationary. Unfortunately we are thinking of
that object as being the target (which it isn't) rather than the moving
charge (which it is). When we find our tower is in the elevated charge
zone, it *may* be the highest charged point, BUT the strongest charge may
not be the tower. It might be no more than a few hundred feet away. All
things being equal height does make a big difference, but things are seldom
equal. The charge which is certainly the most important is not equal, or
even uniform.
I've seen the average tower height dismissed as it is considered
insignificant compared to the length of the lightning bolt. That height can
make a considerable difference, but it may or may not in the real world
under specific conditions.
As an experiment: hook two plates to a Vandegraf Generator and place them
just close enough to each other so that at the maximum charge build up they
will arc over. Now put a couple of sharp points on each of the surfaces.
It depends on how fast the generator can build the charge (coulombs per
second), but corona should develop around the points preventing the plates
from reaching the point where they can arc over. BUT "at times' like in the
real world, if the generator can build the charge faster than it can be bled
off the strike will occur any way.
In that particular case the point does provide a source for a feeder and the
porcupine does provide a discharge, or leakage path for preventing or
slowing the charge build up on the tower. Whether that is a fast enough
discharge rate is do any good is pure conjecture. OTOH blunt objects do
prevent the corona, or leakage current and should allow for the minimum
charging time.
Much, if not most of the information on lightning strikes is antidotal. I
don't have a bunch of pointy objects on top of my tower unless you count the
element ends, in which case I have a lot of them. <:-))
http://www.rogerhalstead.com/ham_files/Tower29.htm This system has been
taking about three verified direct hits a season. It's well past that for
this year with it up to five and it's only the beginning of July. I know it
took two hits a couple nights ago. I think it took three (maybe even more),
but I'm not counting that third one because I don't know for sure.
> No need to present facts available to you. Just look up measured energy
> of
> a strike and compare it to the ability of the porcupine to dissipate this
The measured energy of the stike is not the point here. The point is to try
and prevent the charge build up to where the strike ocurrs. It takes far
less current to beed off a charge or prevent the build up within limits than
is contained in a strike. The point (no pun intended) is to bleed off the
charge faster than the ground charge builds, whether it be moving into the
area, building up, or both. If that charge is building on the object faster
than it can be bled off and a feeder is produced that connects then the
object will be hit. When that strike ocurrs the current comes from an area
considerably larger than the tower and contains far more energy than would
have been available from the tower.
> energy. BTW, while you are looking for facts, try to determine the amount
> of charge that can be dissipated by the porcupine.
>
To be effective the porcupine would most likely have to dissipate very
little energy. That is not its job.
Now, as to whether the porucpine actually does do its job is some thing else
entirely, but has little to do with the arguments presneted here.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com
> 73, Keith NM5G
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Doug Renwick
> Sent: Tuesday, July 04, 2006 9:57 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Grounding base slab and pier tower bases
>
> I did not say that porcupines will stop all lightning strikes.
> I said "In other words, my addition of the porcupine did not have a
> negative
> effect."
> Scientific experiment will only trump experience IF the scientific
> experiment can exacting duplicate the experience and conditions!
> Please present your scientific experiment that proves porcupines will not
> stop strikes, any strike.
>
> Doug
>
> Subject: Re: [TowerTalk] Grounding base slab and pier tower bases
>
> Experience may trump theory, but scientific experiment trumps experience
> EVERY time! porcupines on towers have been shown to not stop strikes.
> Not
> any more than an owl stops birds.
>
>
> David Robbins K1TTT
>
>
> --
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>
>
>
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